Mass Merchant(Walmart,Costco, Etc.)

Wrecking Yard

Online catalogs

other

On parts designed for wear and tear, I usually take the opportunity to upgrade said worn out part with an aftermarket upgraded replacement. ie brakes, plugs, plug wires, suspension components, etc...
However, I do prefer OEM. At least with American Made vehicles.

I purchase for a local parts store, right part everytime. I used them when I was a service manager and customers did not want to pay Chrysler parts prices. These guys have real parts, Moog, Wagner, etc and they purchase direct for the manufacture.

Usually wear parts from the Advance Auto or a local non-chain store; both right around the corner from me. Since we have an account with NAPA and Summit, I could buy there cheap if it'll end up better price. Performance parts I'll buy online - Autoanything, Real Truck, Summit.